1. Field of the Invention
The present invention relates to an image-forming apparatus such as a copying machine having a plurality of charging means for an image carrier and controlling means for controlling the charging means.
2. Description of the Related Art
In a conventional image-forming apparatus such as a copying machine, a rotary photosensitive drum 1 as an image carrier is uniformly charged by one charging member 2 so that an electrostatic latent image is formed thereon by exposing means such as semiconductor laser 7 in accordance with image information. A metallic circular cylinder with an external surface having a photosensitive layer is used as the photosensitive drum 1. Then, the latent image is developed by developing means 3 so as to form a toner image on the photosensitive drum 1. The toner image formed on the photosensitive drum 1 is transferred on a recording sheet by a transferring member 4 and fixed thereon by fusing means 6 so as to have a permanent image. After the transferring of the toner image, the photosensitive drum 1 is finally cleaned by cleaning means 5.
If the surface velocity of the photosensitive drum 1 is increased for improving a printing speed, the surface potential of the photosensitive drum 1 cannot have a desired value with one time charging, so that several times of charging are required to form the latent image, delaying the printing speed.
Then, Japanese Patent Laid-Open No. 8-44153 discloses an image-forming apparatus having a plurality of chargers so as to reduce the charging time.
However, in the above-mentioned image-forming apparatus having a plurality of the chargers, even when the photosensitive drum 1 is charged at the same potential, there is a disadvantage that the power consumption is increased in accordance with the combination of voltages applied to each charger.